Driver circuit with extended operation range

ABSTRACT

A system comprising a driver circuit wherein the driver circuit comprises a converter. The system further comprises a controller operable to control a ratio between an input voltage supplied to the driver circuit and a target output voltage to be output from the driver circuit by varying a pulse width modulated signal, and by switching between operating modes of said converter. The controller is configured to determine that the driver circuit is operating in an undesired state caused by a combination of the duty cycle and the operating mode of the converter. The system further comprises a resistance adjustment circuit configured to adjust an equivalent output resistance of the driver circuit in response to the controller determining that the driver circuit is operating in an undesired state.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/EP2015/053143, filed on Feb.13, 2015, which claims the benefit of European Patent Application No.14164134.0, filed on Apr. 10, 2014 and Chinese Patent Application No.PCT/CN2014/000170, filed on Feb. 24, 2014. These applications are herebyincorporated by reference herein.

TECHNICAL FIELD

The invention relates to the field of driver circuits, and moreparticularly to driver circuits for Light Emitting Diode (LED) lightsources.

BACKGROUND

The Solid State Lighting (SSL) Industry's demand for small and compactpower management units for LEDs is increasing. This demand is evenstronger in the field of Power LEDs, for which energy from the powersupply has to be delivered in the form of a constant current asefficiently as possible.

“Driver on board” technology is known where the SSL light source and thedriver are to be combined on one carrier board. Examples of this may be(i) SSL light sources being LEDs (with or without light conversion, suchas direct, vicinity, remote or any other phosphor), combined with atleast part of the driver on a printed circuit board; (ii) SSL lightsources being LEDs, combined with at least part of the driver on anothercarrier, such as a silicon wafer, glass plate, (thermally enhanced)plastic carrier; (iii) SSL light sources being OLEDs, combined with atleast part of the driver on the carrier of the OLED; (iv) Combination oflaser and driver; and (v) Combination of light source, and other signalor power electronics.

In these “driver on board” scenarios, size is one constraint, either asa limitation in the footprint (the “real estate” of the driver circuitoccupied on the board) and/or the height of the components. Theselimitations may originate from optical constraints, for example highcomponents may block part of the light emission from a light source, orlarge surface components may impact the total reflectivity of a lightmixing chamber.

To one end small size of the driver circuit is a target, however smallsize comes with low volume of components. To the other end highperformance is the target, however low volume of components results inrelatively limited performance, which challenges driver circuit design.

Multimode driver circuits are typically implemented by using a powerconverter. Multimode driver power converters allow regulating a constantoutput voltage with a plurality of ranges of conversion ratios, in orderto cope with the variations of input voltage. The input supply voltagemay be from either an Alternating Current (AC) input (from power grid)or a Direct Current (DC) input (battery, AC rectified voltage etc.).Voltage variations from these sources are unavoidable. For example, whenpower is coming from AC power grid, intrinsic quality of electricalgenerator and transmission loss through the wire will typically resultin a 10% error providing the AC supply voltage to users. Adapters areused to rectify AC voltage to DC voltage and are controlled byelectronic circuits. The performance (DC regulation ability, processvariation, etc.) of these electronic circuits will provide different DCvoltages. For a portable device powered by a lithium battery, thevoltage from the battery will typically vary from 4.2V (fully charged)to 2.7V (fully used). The power conversion ratio is essentiallyproportional to the duty cycle in each operating range, wherein desiredconversion ratio can typically be achieved through adjusting the dutycycle of a Pulse Width Modulation (PWM) signal.

SUMMARY

The inventors have recognised that at the boundaries of each operatingrange of a multimode power converter, efficiency falls rapidly, possiblyto zero, resulting in a very limited performance of the converter incertain areas. Such efficiency “gaps” become a significant disadvantageof multi-mode converters and largely constrain their application.

The invention proposes to fill in such gaps, by covering the totalrequired range of conversion ratios with one topology.

According to one aspect of the present invention there is provided asystem comprising: a driver circuit comprising a voltage converter; acontroller operable to control a ratio between an input voltage suppliedto the driver circuit and a target output voltage to be output from thedriver circuit by varying a duty cycle of a pulse width modulatedsignal, and by switching between operating modes of said voltageconverter; wherein the controller is configured to determine that thedriver circuit is operating in an undesired state caused by acombination of the duty cycle and the operating mode of the converter;and a resistance adjustment circuit configured to adjust an equivalentoutput resistance of the driver circuit in response to saiddetermination.

In embodiments, the driver circuit is operable to provide an outputvoltage at a varying efficiency dependent on the ratio, wherein theundesired state corresponds to a region of a relationship between theefficiency and the radio.

The resistance adjustment circuit may be configured to adjust theequivalent output resistance of the driver circuit by controlling a gatedrive voltage output to switches of the voltage converter.

The resistance adjustment circuit may be configured to adjust theequivalent output resistance of the driver circuit by controlling aswitching frequency of switches of the voltage converter.

The resistance adjustment circuit may be configured to adjust theequivalent output resistance of the driver circuit by controlling a deadtime associated with switches of the voltage converter.

The resistance adjustment circuit may be configured to adjust theequivalent output resistance of the driver circuit based on receiving anerror signal, said error signal generated based on comparing a referencevoltage and a sense voltage.

The controller may be configured to determine that the driver circuit isoperating in an undesired state based on said ratio.

The controller may be configured to determine that the driver circuit isoperating in an undesired state based on the duty cycle of the pulsewidth modulated signal.

The controller may be configured to determine that the driver circuit isoperating in an undesired state based on the input voltage supplied tothe driver circuit.

The controller may be configured to determine that the driver circuit isoperating in an undesired state based on determining the efficiency ofthe driver circuit.

The controller may be configured to constrain the duty cycle of thepulse width modulated signal to be limited within a limited duty cyclerange.

The controller may be configured to constrain the duty cycle of a pulsewidth modulated signal to be at a constant value within the limited dutycycle range in response to determining that the driver circuit isoperating in an undesired state.

The voltage converter may be formed by a switched capacitor convertercomprising a first set of switches and a second set of switches.

The voltage converter may be based on a Dickson ladder topology.

According to another aspect of the present invention there is provided alight module comprising at least one light source and the systemdescribed herein wherein the driver circuit provides an output voltagethat drives said at least one light source.

According to a further aspect of the present invention there is provideda computer program product for controlling a driver circuit comprising avoltage converter, and a resistance adjustment circuit, the computerprogram product comprising code embodied on a computer-readable mediumand being configured so as when executed on a processor to: control aratio between an input voltage supplied to the driver circuit and atarget output voltage to be output from the driver circuit by varying aduty cycle of a pulse width modulated signal, and by switching betweenoperating modes of said voltage converter; determine that the drivercircuit is operating in an undesired state caused by a combination ofthe duty cycle and the operating mode of the converter; and supply thepulse width modulated signal to the resistance adjustment circuit toadjust an equivalent output resistance of the driver circuit in responseto said determination.

These and other aspects will be apparent from the embodiments describedin the following. The scope of the present disclosure is not intended tobe limited by this summary nor to implementations that necessarily solveany or all of the disadvantages noted.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present disclosure and to show howembodiments may be put into effect, reference is made to theaccompanying drawings in which:

FIG. 1 illustrates a typical curve of efficiency with conversion ratioand duty cycle of a multimode converter;

FIG. 2 shows a schematic block diagram of an apparatus according to anexemplary embodiment of the present invention;

FIG. 3 shows an LED driver circuit according to an exemplary embodimentof the present invention;

FIG. 4 illustrates an equivalent circuit of a voltage converter;

FIG. 5 illustrates a charge process of a first operating mode of the LEDdriver circuit shown in FIG. 4;

FIG. 6 illustrates a charge process of a second operating mode of theLED driver circuit shown in FIG. 4;

FIG. 7 illustrates a charge process of a third operating mode of the LEDdriver circuit shown in FIG. 4;

FIG. 8 illustrates a charge process of a fourth operating mode of theLED driver circuit shown in FIG. 4;

FIG. 9 illustrates a process according to an exemplary embodiment of thepresent invention to extend the operating range of a driver circuit;

FIG. 10 shows the result of efficiency enhancement by using the processillustrated in FIG. 9; and

FIG. 11 illustrates how a continuous output voltage is output from theLED driver circuit shown in FIG. 3 according to exemplary embodiments ofthe present invention.

DETAILED DESCRIPTION

For a driver-on-board power converter, a fundamental requirement is theability to operate in multiple modes, which can cover various electricalspecifications e.g. input or output voltage, conversion ratio,efficiency, etc.

Switched Mode Power Supplies (SMPS) driver circuits using capacitiveenergy storage—SMPS notably including Switched Capacitor (SC)Converters, also called Charge Pumps (CP)—can provide highly efficientDC-to-DC voltage conversion with only the use of capacitors andsemiconductor switches. An SCC converter may produce a regulated outputvoltage by either stepping up or stepping down the input voltage.

The efficiency, η, of an SC converter can be expressed as function ofthe voltage transfer ratio independent of switch resistances:

$\eta = {\frac{V_{o}}{M \cdot V_{in}} = \frac{V_{o}}{V_{T}}}$where V_(o) is the output voltage of the converter, V_(in) is the inputvoltage of the converter, V_(T) is the target voltage and M is theconversion ratio.

A multimode power converter may have multiple desired operatingconditions, which are separated by undesired operating conditions. Inparticular, a multimode power converter may be able to supply power in adesired fashion to at least one load coupled to least one output of theconverter when the voltage, required by the load is either in rangeV_(o1) or in range V_(o2), with:V _(a) >V _(o1) >V _(b)V _(c) >V _(o2) >V _(d)wherein V_(b)>V_(b).

So, when the voltage required by the load is in the range V_(o3) where:V _(b) <V _(o3) <V _(c)the converter may not operate at all, or only with very limitedperformance.

These voltage “gaps” cause the efficiency of the SC converter to fallrapidly, even to zero.

FIG. 1 shows a typical efficiency curve 102 and duty cycle curve 104with conversion ratio of a step down SC power converter which isdesigned with four operating modes which can provide four ranges ofconversion ratio. This enables the power converter to be applied in widevoltage range applications.

Functionally, when regulating a constant output voltage, four ranges ofconversion ratio (0.2 to 0.4, 0.4 to 0.6, 0.6 to 0.8 and 0.8 to 1) areavailable to meet the variation of input voltage. Therefore, outputvoltage could be easily chosen from ranges below:0.2V _(in) <V _(o1)<0.4V0.4V _(in) <V _(o2)<0.6V _(in)0.6V _(in) <V _(o3)<0.8V _(in)0.8V _(in) <V _(o4) <V _(in)

As shown in FIG. 1, the conversion ratio is proportional to the dutycycle with each operating mode. Thus, tuning the duty cycle e.g. usingPulse Width Modulation (PWM) is the general method to achieve a desiredconversion ratio.

However, at the boundaries of each operating mode, where the targetconversion ratio is 0.2, 0.4, 0.6 and 0.8, it can be seen thatefficiency falls rapidly, even to zero. Thus the power converterachieves very limited performance at these boundaries. It can be seenfrom FIG. 1 that the efficiency falls rapidly when the duty cycleapproaches zero and when the duty cycle approaches one.

These efficiency “gaps” are closed by embodiments of the presentinvention. That is, embodiments of the present invention extend theoperating range of a driver circuit. Reference is now made to FIG. 2which illustrates a system 200. The system 200 comprises a power supply20, which supplies an input voltage V_(in) to a driver circuit 201.

In the example system 200 shown in FIG. 2, the driver circuit 201 isused to drive an LED (D). However embodiments are not limited to onlyaddressing driving of light loads, but also other applications, whereinsize constraints similarly apply (e.g. in devices such as smart phones,laptop PCs and other portable devices, which need slim and compactelectronic design).

An example driver circuit 201 is shown in FIG. 3. As shown in FIG. 3,the driving unit 201 can comprise a voltage converter 300, for example acharge pump that can be based on a SC Converter, for example of theLadder topology type. The driving unit 201 is supplied by an inputvoltage V_(in) supplied by the power supply 20, which can be part of theLED driver or external to the LED driver.

In the exemplary embodiment illustrated by FIG. 3, the driving unit 201comprises a so-called Dickson Ladder converter forming the voltageconverter 300. It shall be observed that other SCC topologies can beused, such as standard ladder, Fibonacci, or series-parallel topologiesfor instance.

The illustrated exemplary embodiment uses a Dickson Ladder topologybased on five capacitors C1 to C5 and nine switches S1 to S9 of thesingle pole, single throw type. More specifically, the driving unit 201comprises two flying ladders, each flying ladder comprising twocapacitors: one first flying ladder comprises a first capacitor C1 inseries with a third capacitor C3, and a second flying ladder comprises asecond capacitor C2 in series with a fourth capacitor C4.

The driving unit 201 further comprises six central nodes 3011 to 3016. Afirst switch S1 selectively connects the first central node 3011 to thepositive terminal of the power supply 20. A second switch S2 selectivelyconnects the first central node 3011 to the second central node 3012. Athird switch S3 selectively connects the second node 3012 to the thirdcentral node 3013. A fourth switch S4 selectively connects the thirdcentral node 3013 to the fourth central node 3014. A fifth switch S5selectively connects the fourth central node 3014 to the fifth centralnode 3015. A fifth capacitor S5 is placed between the fifth central node3015 and one sixth central node 3016. The sixth central node 3016 isconnected to the negative terminal of the power supply 20.

The first flying ladder comprising the first and third capacitors C1, C3is located between the first central node 3011 and a first secondarynode 3021. A sixth switch S6 selectively connects the first secondarynode 3021 to the fifth central node 3015; a seventh switch S7selectively connects the first secondary node 3021 to the sixth centralnode 3016. A node in-between first capacitor C1 and third capacitor C3is connected to the third central node 3013.

The second flying ladder comprising the second and fourth capacitors C2,C4 is located between the second central node 3012 and a secondsecondary node 3022. An eighth switch S8 selectively connects the secondsecondary node 3022 to the fifth central node 3015; a ninth switch S9selectively connects the second secondary node 3022 to the sixth centralnode 3016. A node in-between second capacitor C2 and fourth capacitor C4is connected to the fourth central node 3014.

The two flying ladders are oppositely phased, thanks to an adequatesequence of opening and closing the switches S1 to S9.

The controller 202 (shown in FIG. 2) is configured to generate a firstPWM signal to control switches S1, S3, S5, S6, and S9 of the drivercircuit 201, and a second PWM signal to control switches S2, S4, S7, andS8 of the driver circuit 201. The first PWM signal is used to controlswitches S1, S3, S5, S6, and S9 to be in a given state during a firsttime phase ϕ1, for instance turned on, while the second PWM signal isused to control switches S2, S4, S7, and S8 to be in the opposite state,for instance turned off. During a successive second time phase ϕ2, thestates of all the switches can be reversed. That is, first PWM signalhas a duty cycle D, and the second PWM signal has a duty cycle of 1−D.The first PWM signal and the second PWM signal generated by thecontroller 202 may be supplied to the driver circuit 201 (as shown byconnection 206 a). Alternatively the first PWM signal and the second PWMsignal generated by the controller 202 may be supplied to a resistanceadjustment circuit 210, as will be described in more detail below.

The driver circuit 201 can further comprise a multiplexer 310 formed byfour multiplexer switches S10, S11, S12, S13 connected to the first fourcentral nodes 3011, 3012, 3013, 3014 respectively. The multiplexer 310selectively connects one of the internal nodes of the voltage converter300, that is: either one of the first four central nodes 3011, 3012,3013, 3014, one at a time, outputting a driver voltage as a function ofrespective intermediate voltages at the first four central nodes 3011,3012, 3013, 3014 respectively.

The multiplexer 310 selectively closes one of the multiplexer switchesS10, S11, S12, S13 based on a mode selection control signal 204 receivedfrom the controller 202. Thus the multiplexer 310 controls the operatingmode of the driver circuit 201.

The output of the multiplexer 310 can be connected to a filter inductorL₁ placed in series with the LED D. A capacitor C_(o) can be placed inparallel with the LED D and a load resistor R_(L). The output filterformed by the filter inductor L₁, the capacitor C_(o), and the loadresistor R_(L) filters the internal pulsating voltages of the SCconverter, supplying a constant current to the LED D.

As shown in FIG. 2, a sense voltage V_(sense) is supplied to acomparator 208 which compares the sense voltage V_(sense) with areference voltage V_(ref) to generate an error signal 209 which issupplied to the resistance adjustment circuit 210. The reference voltageV_(ref) may be set to the output voltage that is required for a certainapplication by an end-user. As a mere example, the reference voltageV_(ref) may be set to for example 5V or 3.3V. In other cases, thereference voltage V_(ref) may be set to a fixed reference voltage, forexample 1.2V, and an adjustable resistor ladder provided, the resistanceof which can be adjusted by an end-user according to the output voltagethat is required for a certain application. The sense voltage V_(sense)is proportional to, and thus provides a measure of, the output currentI_(o) of the driver circuit 201. The error signal 209 is generated bythe comparator 208 by subtracting the sense voltage V_(sense) from thereference voltage V_(ref). The comparator 208 may be implemented usingfor example an operational amplifier. The comparator 208 supplies theerror signal 209 to the resistance adjustment circuit 210.

As shown in FIG. 2, the controller 202 receives the input voltage V_(in)as a first input signal 214, the target voltage V_(T) as a second inputsignal 216, and the output voltage V_(o) as a third input signal 218.Based on the input voltage V_(in) and the target voltage V_(T), thecontrol module is configured to dynamically determine a requiredconversion ratio M (where M=V_(T)/V_(in)). The controller 202 isconfigured to dynamically determine a mode of operation and duty cycle Dthat is needed to provide the required conversion ratio M (theconversion ratio M is proportional to the duty cycle with each operatingmode). It will be appreciated that the controller 202 may bepre-configured with the target voltage V_(T), and in such scenarios thecontroller 202 may not receive the target voltage V_(T) using the secondinput signal 216.

An equivalent circuit of the driver circuit 201 is shown in FIG. 4. Asshown in FIG. 4, the driver circuit 201 can be modelled, as anequivalent voltage source, defined as the target voltage V_(T),connected in series with an equivalent output resistance (R_(out)) thatrepresents the internal power losses of the driving circuit 201 due topower dissipation in switch resistances and capacitors' equivalentseries resistance (ESR). The output voltage V_(o) of the driver circuit201 can be expressed as:V _(o) =V _(T) −Rout*I _(L)

FIGS. 5-8 shows the charge process of each operating mode of the drivercircuit 201.

In particular FIGS. 5a-c shows the driver circuit 201 operating in afirst operating mode, wherein S10 of the multiplexer 310 is closed andswitches S11, S12, S13 of the multiplexer 310 are open, to provide anoutput voltage V_(o1). In FIG. 5a all of switches S1-S9 are open. FIG.5b illustrates the charge contribution of capacitors and switches withregards to V_(o1) during a first time phase ϕ1 when the first PWM signalcloses switches S1, S3, S5, S6, and S9, and the second PWM signal opensswitches S2, S4, S7, and S8. FIG. 5c illustrates the charge contributionof capacitors and switches with regards to V_(o1) during a second timephase ϕ2 when the first PWM signal opens switches S1, S3, S5, S6, andS9, and the second PWM signal closes switches S2, S4, S7, and S8.

FIGS. 6a-c shows the driver circuit 201 operating in a second operatingmode, wherein S11 of the multiplexer 310 is closed and switches S10,S12, S13 of the multiplexer 310 are open, to provide an output voltageV_(o2). In FIG. 6a all of switches S1-S9 are open. FIG. 6b illustratesthe charge contribution of capacitors and switches with regards toV_(o2) during a first time phase ϕ1 when the first PWM signal closesswitches S1, S3, S5, S6, and S9, and the second PWM signal opensswitches S2, S4, S7, and S8. FIG. 6c illustrates the charge contributionof capacitors and switches with regards to V_(o2) during a second timephase ϕ2 when the first PWM signal opens switches S1, S3, S5, S6, andS9, and the second PWM signal closes switches S2, S4, S7, and S8.

FIGS. 7a-c shows the driver circuit 201 operating in a third operatingmode, wherein S12 of the multiplexer 310 is closed and switches S10,S11, S13 of the multiplexer 310 are open, to provide an output voltageV_(o3). In FIG. 7a all of switches S1-S9 are open. FIG. 7b illustratesthe charge contribution of capacitors and switches with regards toV_(o3) during a first time phase ϕ1 when the first PWM signal closesswitches S1, S3, S5, S6, and S9, and the second PWM signal opensswitches S2, S4, S7, and S8. FIG. 7c illustrates the charge contributionof capacitors and switches with regards to V_(o3) during a second timephase ϕ2 when the first PWM signal opens switches S1, S3, S5, S6, andS9, and the PWM signal closes switches S2, S4, S7, and S8.

FIGS. 8a-c shows the driver circuit 201 operating in a fourth operatingmode, wherein S13 of the multiplexer 310 is closed and switches S10,S11, S12 of the multiplexer 310 are open, to provide an output voltageV_(o4). In FIG. 8a all of switches S1-S9 are open. FIG. 8b illustratesthe charge contribution of capacitors and switches with regards toV_(o4) during a first time phase ϕ1 when the first PWM signal closesswitches S1, S3, S5, S6, and S9, and the second PWM signal opensswitches S2, S4, S7, and S8. FIG. 8c illustrates the charge contributionof capacitors and switches with regards to V_(o4) during a second timephase ϕ2 when the first PWM signal opens switches S1, S3, S5, S6, andS9, and the second PWM signal closes switches S2, S4, S7, and S8.

The output voltage in each of the four operating modes referred to abovecan be expressed as:V _(o1) =V _(in) *M ₁ −R _(out1) *I _(L)V _(o2) =V _(in) *M ₂ −R _(out2) *I _(L)V _(o3) =V _(in) *M ₃ −R _(out3) *I _(L)V _(o4) =V _(in) *M ₄ −R _(out4) *I _(L)

Where V_(o(n)) is the output voltage at operating mode n, M_((n)) is theconversion ratio at operating mode n, R_(out(n)) is the equivalentoutput resistance at operating mode n, and I_(L) is the load current.

As shown in the formulas above, a specific output voltage is dependenton the conversion ratio, load current and equivalent output resistance.

By using Kirchhoff Charge Law (KQL), the output voltage of eachoperating mode can be written as:V _(o1)=⅘V _(IN)+⅕V _(IN) D−α ₁(D)R _(ds,on) I _(o)V _(o2)=⅘V _(IN)−⅕V _(IN) D−α ₂(D)R _(ds,on) I _(o)V _(o3)=⅖V _(IN)+⅕V _(IN) D−α ₃(D)R _(ds,on) I _(o)V _(o4)=⅖V _(IN)−⅕V _(IN) D−α ₄(D)R _(ds,on) I _(o)

Where, V_(in) is the input voltage, D is the duty cycle, R_(ds, on) isthe turn-on resistance of the switches S1-S9 of the voltage converter300, and I_(o) is the output current, respectively. Coefficient α(D) is:

${\alpha_{1}(D)} = \frac{{{- 2}\; D^{3}} + {25\; D^{2}} - {71\; D} + 47}{25\;{D\left( {1 - D} \right)}}$${\alpha_{2}(D)} = \frac{{- D^{3}} + {14\; D^{2}} - {29\; D} + 31}{25\;{D\left( {1 - D} \right)}}$${\alpha_{3}(D)} = \frac{{- D^{3}} + {73\; D^{2}} - {55\; D} + 34}{25\;{D\left( {1 - D} \right)}}$${\alpha_{4}(D)} = \frac{{- D^{3}} + {10\; D^{2}} - {33\; D} + 34}{25\;{D\left( {1 - D} \right)}}$

From the formulas above, it can be seen that the output voltage andequivalent output resistance (R_(out)) are both duty cycle dependant.When the duty cycle is close to the boundaries, e.g. 0 or 1, the outputvoltage will reduce to zero and the equivalent output resistance will beinfinite, resulting in extremely limited performance of the drivercircuit 201.

Known control strategies of converters will often minimize R_(out), inorder to reduce the losses and/or to provide a stable, load-independentoutput signal.

Reference is now made to a process 900 implemented by the presentinvention to close the above described efficiency gaps to extend theoperating range of the driver circuit 201. The process 900 starts atstep S902 and proceeds to step S904 where the controller 202 determineswhether the driver circuit 201 is operating in an undesired state causedby a combination of the duty cycle, D and the operating mode of thevoltage converter 300.

The controller 202 may employ various methods to determine whether thedriver circuit 201 is operating in a desired or undesired state.

The controller 202 is configured to determine a required conversionratio, M based on the input voltage, V_(in) (received via first inputsignal 214) and the target voltage, V_(T) (received via second inputsignal 216). In one embodiment, the controller 202 makes thedetermination at step S904 by comparing the required conversion ratio, Mis compared with known conversion ratio ranges associated with thedriver circuit 201 operating in desired and undesired states. Thisenables the controller 202 to determine whether the required conversionratio, M is associated with a desired or an undesired operating state ofthe driver circuit 201. The information relating to the known conversionratio ranges associated with desired and undesired operating states ofthe driver circuit 201 may be stored in a memory (not shown in FIG. 2)coupled to the controller 202

The controller 202 is configured to determine a duty cycle, D to achievethe required conversion ratio, M. In an alternative embodiment, thecontroller 202 makes the determination at step S904 by comparing theduty cycle, D with known duty cycle ranges that are associated with thedriver circuit 201 operating in desired and undesired states based onthe driver circuit 201 being configured to provide a target voltageV_(T). This enables the controller 202 to determine whether the dutycycle, D is associated with a desired or an undesired operating state ofthe driver circuit 201. The information relating to the known dutycycle, D ranges associated with desired and undesired operating statesof the driver circuit 201 may be stored in the memory referred to above.

In a further embodiment, the controller 202 makes the determination atstep S904 by detecting the input voltage V_(in) (received via firstinput signal 214) and comparing the input voltage V_(in) with knowninput voltage ranges that are associated with the driver circuit 201operating in desired and undesired states based on the driver circuit201 being configured to provide a target voltage V_(T). The informationrelating to the input voltage ranges associated with desired andundesired operating states of the driver circuit 201 may be stored inthe memory referred to above.

In yet a further embodiment, the controller 202 is configured todynamically determine the efficiency of the driver circuit 201 based onthe target voltage, V_(T) (received via second input signal 216) and theoutput voltage V_(o) (received via third input signal 218). Thecontroller 202 is configured to make the determination at step S904 bycomparing the determined efficiency to a predetermined thresholdefficiency level, and if the determined efficiency is greater than, orequal to, the predetermined threshold efficiency level, the controller202 determines that the driver circuit 201 is operating in a desiredstate, otherwise the the controller 202 determines that the drivercircuit 201 is operating in an undesired state.

If the controller 202 determines that the driver circuit 201 isoperating in a desired state at step S904 (i.e. not an undesired state),the process 900 proceeds to step S906.

At step S906, the controller 202 supplies the generated first PWM signaland second PWM signal to the appropriate switches of the driver circuit201 (as shown by connection 206 a in FIG. 2). Whilst the driver circuit201 is operating in a desired state, the controller adjusts the dutycycle, D to achieve to achieve the required conversion ratio, M. Toavoid operating in an undesired region, the controller 202 limits theduty cycle, D within a limited duty cycle range. The upper boundary ofthe limited duty cycle range being smaller than 1 and the lower boundaryof the duty cycle range being greater than 0. For example the limitedduty cycle range may be limited to 0.2≤D≤0.8. It will be appreciatedthat this limited duty cycle range is merely an example.

Referring back to step S904, if the controller 202 determines that thedriver circuit 201 is operating in an undesired region of an operatingmode, the process 900 proceeds to step S908.

At step S908, the controller 202 supplies the generated first PWM signaland second PWM signal to the resistance adjustment circuit 210 (as shownby connection 206 b in FIG. 2) and the resistance adjustment circuit 210adjusts the equivalent output resistance, R_(out), of the driver circuit201 to enhance the efficiency of the driver circuit 201.

From the formulas above it can be seen that the equivalent outputresistance, R_(out), is a function of the duty cycle and the R_(ds, on)of the switches S1-S9.

In one embodiment, the resistance adjustment circuit 210 comprises gatedrivers to provide a gate drive voltage V_(g) to each of the switches.The gate drivers associated with switches S1, S3, S5, S6, and S9 areconfigured to receive as an input the first PWM signal from thecontroller 202, whereas the gate drivers associated with switches S2,S4, S7, and S8 are configured to receive as an input the second PWMsignal from the controller 202. The gate drivers are configured tooutput the PWM signal that they receive to the gate terminal of itsassociated switch. The PWM signal output from each gate driver has anoutput voltage amplitude corresponding to the voltage source of the gatedriver (V_(GS)). As will be appreciated by persons skilled in the artgate drivers may take many forms, one of the most popular andcost-effective gate driver is a bipolar, non-inverting totem-poledriver.

The R_(ds,on) of the switches S1-S9 is a function of V_(GS) of the gatedrivers used to provide the PWM signals to the switches S1-S9. In thisembodiment, the resistance adjustment circuit 210 controls the gatedrive voltage output to switches of the voltage converter 300 bycontrolling V_(GS) of the gate drivers to adjust the equivalent outputresistance, R_(out) of the voltage converter 300 and thus the drivercircuit 201. For example, the resistance adjustment circuit 210 maycomprise a shunt regulator configured to receive the error signal 209 atits reference terminal and regulate V_(GS). As will be known to personsskilled in the art, increasing the gate drive voltage input to a switch,decreases the R_(ds, on) of the switch.

In another embodiment, the resistance adjustment circuit 210 adjusts theswitching frequency of the switches S1-S9 to adjust the equivalentoutput resistance, R_(out) of the voltage converter 300 and thus thedriver circuit 201. For example, the resistance adjustment circuit 210may comprise a voltage controlled oscillator (VCO) configured to receivethe error signal 209. Thus the oscillation frequency of the VCO iscontrolled by the error signal 209. Logic circuitry may be used tocombine the first PWM signal received from the controller 202 with thevoltage controlled oscillator output signal received from the VCO toprovide a driver signal for the switches S1, S3, S5, S6, and S9.Similarly, this logic circuitry may be used to combine the second PWMsignal received from the controller 202 with the voltage controlledoscillator output signal received from the VCO to provide a driversignal for the switches S2, S4, S7, and S8. By increasing the switchingfrequency of the switches S1-S9 the resistance adjustment circuit 210decreases the equivalent output resistance, R_(out) of the voltageconverter 300. The operation of the voltage converter, for exampleformed by a switched capacitor converter, can be modelled with twooperating modes, delimited by a switching frequency limit: a fastswitching mode and a slow switching mode. The switching frequency limitf_(s) can be expressed as:f _(s)=3·Ron*C _(f),where Ron is the turn-on resistance of the switches and C_(f) is thecapacitance of the flying capacitors.

When the switching frequency is below the switching frequency limitf_(s), then for every cycle, the flying capacitors can be fully charged,so the voltage converter operates in so-called slow switching mode. Inthis mode, the equivalent output resistance can be significantly reducedby increasing the switching frequency. When the switching frequency isabove the switching frequency f_(s), then for every cycle, the flyingcapacitors cannot be fully charged, and the voltage converter operatesin so-called fast switching mode. In this mode, the equivalent outputresistance can be gradually reduced by increasing the switchingfrequency.

In another embodiment, the resistance adjustment circuit 210 comprisescircuitry to adjust the dead time associated with the voltage converter300 to adjust the equivalent output resistance, R_(out) of the voltageconverter 300 and thus the driver circuit 201. The dead time of thevoltage converter 300 refers to the time period during a switchingtransition when the first set of switches (S1, S3, S5, S6, S9) and thesecond set of switches (S2, S4, S7, and S8) are both are open. Byincreasing the dead time associated with the voltage converter 300 theresistance adjustment circuit 210 increases the equivalent outputresistance, R_(out) of the voltage converter 300.

At step S908 the controller 202 limits the duty cycle, D within thelimited duty cycle range referred to above. In particular, thecontroller 202 may keep the duty cycle, D constant within the limitedduty cycle range. For example the controller 202 may set the duty cycle,D to be at the lowest possible duty cycle in the limited duty cyclerange (0.2 in the example provided above), at the highest possible dutycycle in the limited duty cycle range (0.8 in the example providedabove), or at a constant duty cycle between the lowest possible dutycycle and the highest possible duty cycle in the limited duty cyclerange—this is application dependent.

FIG. 10 shows the result of efficiency enhancement by using the process900. Four operating modes are divided into seven states, where D1, D2,D3 and D4 are regarded as desired states of operation of the drivercircuit 201, and R1, R2, R3 are regarded as undesired states ofoperation of the driver circuit 201. As described above, whether thedriver circuit 201 is operating in a desired or undesired operatingstate is classified by an operating condition such as efficiency,conversion ratio, duty cycle etc.

Looking at FIG. 10 from right to left it can be seen that whenregulating a constant output voltage, as the input voltage increasesfurther from the required output voltage whilst the driver circuit 201is operating in a desired state D4 of the fourth operating the requiredconversion ratio, M is decreased by decreasing the duty cycle of thefirst PWM signal and the second PWM signal within a limited duty cyclerange (referred to above). Whilst in the desired state D4, thecontroller 202 is configured to supply the first PWM signal and thesecond PWM signal to the driver circuit 201 (shown by connection 206 a).

At edge point 1002 a the controller 202 determines that the drivercircuit 201 is operating in an undesired state of the fourth operatingmode and supplies the first PWM signal and the second PWM signal to theresistance adjustment circuit 210 (shown by connection 206 b). Theadjustment of the equivalent output resistance of the driver circuit 201is implemented by the resistance adjustment circuit 210 until edge point1004 a is reached where the controller 202 determines that the drivercircuit 201 is operating in a desired state D3 of the third operatingmode.

As the input voltage increases further from the required output voltagewhilst the driver circuit 201 is operating in a desired state D3 therequired conversion ratio, M is decreased further by decreasing the dutycycle of the first PWM signal and the second PWM signal within a limitedduty cycle range (referred to above). Whilst in the desired state D3,the controller 202 is configured to supply the first PWM signal and thesecond PWM signal to the driver circuit 201 (shown by connection 206 a).

At edge point 1002 b the controller 202 determines that the drivercircuit 201 is operating in an undesired state of the third operatingmode and supplies the first PWM signal and the second PWM signal to theresistance adjustment circuit 210 (shown by connection 206 b). Theadjustment of the equivalent output resistance of the driver circuit 201is implemented by the resistance adjustment circuit 210 until edge point1004 b is reached where the controller 202 determines that the drivercircuit 201 is operating in a desired state D2 of the second operatingmode.

As the input voltage increases further from the required output voltagewhilst the driver circuit 201 is operating in a desired state D2 therequired conversion ratio, M is decreased further by decreasing the dutycycle of the first PWM signal and the second PWM signal within a limitedduty cycle range (referred to above). Whilst in the desired state D2,the controller 202 is configured to supply the first PWM signal and thesecond PWM signal to the driver circuit 201 (shown by connection 206 a).

At edge point 1002 c the controller 202 determines that the drivercircuit 201 is operating in an undesired state of the second operatingmode and supplies the first PWM signal and the second PWM signal to theresistance adjustment circuit 210 (shown by connection 206 b). Theadjustment of the equivalent output resistance of the driver circuit 201is implemented by the resistance adjustment circuit 210 until edge point1004 c is reached where the controller 202 determines that the drivercircuit 201 is operating in a desired region D1 of the second operatingmode.

As shown in FIG. 10, the adjustment of the equivalent output resistanceby the resistance adjustment circuit 210 enables a desired outputvoltage to be output from the driver circuit 201 given a particularinput voltage and thus extends the range of conversion ratios that canbe used by the voltage converter 300 without a significant loss inefficiency.

The exemplary mode of operation described hereafter, still withreference to FIG. 10, provides a better understanding on how the drivercircuit 201 allow achieving output voltages by either adjusting dutycycle in desired regions or equivalent output resistance in undesiredregions. As described above with reference to FIG. 2, the controller 202receives the input voltage V_(in) as a first input signal 214, thetarget voltage V_(T) as a second input signal 216, and the outputvoltage V_(o) as a third input signal 218. Based on the input voltageV_(in) and the target voltage V_(T), the control module is configured todynamically determine a required conversion ratio M (whereM=V_(T)/V_(in)). The controller 202 is configured to dynamicallydetermine a mode of operation and duty cycle D that is needed to providethe required conversion ratio M (the conversion ratio M is proportionalto the duty cycle with each operating mode). It will be appreciated thatthe controller 202 may be pre-configured with the target voltage V_(T),and in such scenarios the controller 202 may not receive the targetvoltage V_(T) using the second input signal 216. In accordance with therequired range of the conversion ratio M, the controller 202 candetermine in which operation mode the driver circuit 201 shall operate.

In the non-limiting example illustrated by FIG. 10, if the requiredconversion ratio M is between 0.84 and 0.96, then the driver circuit 201shall operate in the fourth desired region D4, thereby adjusting theconversion ratio through varying the duty cycle. If the requiredconversion ratio M is between 0.76 and 0.84, then the driver circuit 201shall operate in the third undesired mode R3, wherein the switches S1 toS13 of the driver circuit 201 are operated in a way similar as describedabove with reference to FIG. 2, i.e. in a way similar as in the fourthdesired region D4, but while having the resistance adjustment circuit210 increasing the equivalent output resistance, for instance byreducing V_(GS) voltage, or by decreasing switching frequency, orincreasing the dead time associated with the voltage converter. If therequired conversion ratio is between 0.64 and 0.76, then the drivercircuit 201 shall operate in the third desired region D3, therebyadjusting the conversion ratio through varying the duty cycle. If therequired conversion ratio M is between 0.56 and 0.64, then the drivercircuit 201 shall operate in the second undesired mode R2, wherein theswitches S1 to S13 of the driver circuit 201 are operated in a waysimilar as described above with reference to FIG. 2, i.e. in a waysimilar as in the third desired region D3, but while having theresistance adjustment circuit 210 increasing the equivalent outputresistance, for instance by reducing V_(GS) voltage, or by decreasingswitching frequency, or increasing the dead time associated with thevoltage converter. If the required conversion ratio is between 0.44 and0.56, then the driver circuit 201 shall operate in the second desiredregion D2, thereby adjusting the conversion ratio through varying theduty cycle. If the required conversion ratio M is between 0.36 and 0.44,then the driver circuit 201 shall operate in the first undesired modeR1, wherein the switches S1 to S13 of the driver circuit 201 areoperated in a way similar as described above with reference to FIG. 2,i.e. in a way similar as in the second desired region D2, but whilehaving the resistance adjustment circuit 210 increasing the equivalentoutput resistance, for instance by reducing V_(GS) voltage, or bydecreasing switching frequency, or increasing the dead time associatedwith the voltage converter. If the required conversion ratio is between0.24 and 0.36, then the driver circuit 201 shall operate in the firstdesired region D1, thereby adjusting the conversion ratio throughvarying the duty cycle.

FIG. 11, illustrates how the combined equivalent output resistanceadjustment and PWM control in accordance with embodiments of the presentinvention can achieve a continuous output voltage. That is, given aninput voltage (e.g. 25V) as the conversion ratio is varied a continuousoutput voltage is output from the driver circuit 201, which wouldotherwise experience significant drops in output voltage (and thussignificant drop in efficiency) during the undesired regions.

As shown by FIG. 11, the minimum efficiency (determined by the lowestcross point) is enhanced from 0% (as shown in FIG. 1) to 78%, meanwhilethe operation range has a 20% extension, from 100% to 120%.

All the switches S1 to S13 as in the illustrated exemplary embodimentcan be bi-directional and implemented in a suitable technology that iscompatible with the switching frequency of the circuit. For instance theswitches can be formed by Metal Oxide Semiconductor Field EffectTransistors (MOSFET) on a silicon substrate or High Electron MobilityTransistors (HEMT) on a Gallium-Nitride substrate.

All the reactive elements can be sized small enough to enableintegration, for example as a Power System on a Chip (PSoC) or PowerSystem in a Package (PSiP). In the illustrated exemplary embodiment, thecapacitance for all capacitors can be set to 100 nF; the capacitancevalues can be further optimized in order to achieve a betterperformance. The inductance value of the filter inductor Lo can be setto 200 nH, making it possible to be printed in an LED sub-mount with asurface of 5×5 mm.

An LED as per any of the above described embodiments could also be a setof LEDs, for example a string of LEDs.

Whilst an example driver circuit is shown in FIG. 3, this is merely anexample and embodiments of the invention are applicable to other drivercircuits that comprise a SC power converter.

Whilst it is described above that the controller 202 is configured togenerate the first PWM signal and second PWM signal (both supplied onconnections 206 a and 206 b), these may be generated by a dedicatedpulse width modulator based on receiving a duty cycle control signalfrom the controller 202.

Whilst embodiments of the present invention have been described abovewith respect to a driver circuit comprising a step-down voltageconverter, embodiments of the present invention also apply to drivercircuit comprising a step-up voltage converter.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. A single processor or other unit may fulfil thefunctions of several items recited in the claims. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measured cannot be used toadvantage. A computer program may be stored/distributed on a suitablemedium, such as an optical storage medium or a solid-state mediumsupplied together with or as part of other hardware, but may also bedistributed in other forms, such as via the Internet or other wired orwireless telecommunication systems. Any reference signs in the claimsshould not be construed as limiting the scope.

The invention claimed is:
 1. A system comprising: a driver circuitcomprising switched capacitor a voltage converter having multipleoperating modes; a controller operable to control a ratio between aninput voltage supplied to the driver circuit and a target output voltageto be output from the driver circuit by varying a duty cycle of a pulsewidth modulated signal, and by switching between operating modes of saidvoltage converter; wherein the controller is configured to determinethat the driver circuit is operating in an undesired state caused by acombination of the duty cycle and the operating mode of the convertersaid undesired state corresponding to an efficiency below a thresholdefficiency level; and a resistance adjustment circuit configured toadjust an equivalent output resistance of the driver circuit in responseto said determination.
 2. The system according to claim 1, wherein thedriver circuit is operable to provide an output voltage at a varyingefficiency dependent on said ratio, wherein the undesired statecorresponds to a region of a relationship between the efficiency andsaid ratio.
 3. The system according to claim 1, wherein the resistanceadjustment circuit is configured to adjust the equivalent outputresistance of the driver circuit by controlling a gate drive voltageoutput to switches of the voltage converter.
 4. The system according toclaim 1, wherein the resistance adjustment circuit is configured toadjust the equivalent output resistance of the driver circuit bycontrolling a switching frequency of switches of the voltage converter.5. The system according to claim 1, wherein the resistance adjustmentcircuit is configured to adjust the equivalent output resistance of thedriver circuit by controlling a dead time associated with switches ofthe voltage converter.
 6. The system according to claim 1, wherein theresistance adjustment circuit is configured to adjust the equivalentoutput resistance of the driver circuit based on receiving an errorsignal, said error signal generated based on comparing a referencevoltage and a sense voltage.
 7. The system according to claim 1, whereinthe controller is configured to determine that the driver circuit isoperating in an undesired state based on said ratio.
 8. The systemaccording to claim 1, wherein the controller is configured to determinethat the driver circuit is operating in an undesired state based on theduty cycle of the pulse width modulated signal.
 9. The system accordingto claim 1, wherein the controller is configured to determine that thedriver circuit is operating in an undesired state based on the inputvoltage supplied to the driver circuit.
 10. The system according toclaim 1, wherein the controller is configured to determine that thedriver circuit is operating in an undesired state based on determiningthe efficiency of the driver circuit.
 11. The system according to claim1, wherein the controller is configured to constrain the duty cycle ofthe pulse width modulated signal to be limited within a limited dutycycle range.
 12. The system according to claim 1, wherein the controlleris configured to constrain the duty cycle of a pulse width modulatedsignal to be at a constant value within said limited duty cycle range inresponse to determining that the driver circuit is operating in anundesired state.
 13. The system according to claim 1, wherein saidvoltage converter is formed by a switched capacitor converter based on aDickson ladder topology comprising a first set of switches and a secondset of switches.
 14. A light module comprising at least one light sourceand the system according to claim 1, wherein the driver circuit providesan output voltage that drives said at least one light source.
 15. Anon-transitory computer program product for controlling a driver circuitcomprising a voltage converter, and a resistance adjustment circuit, thecomputer program product comprising code embodied on a computer-readablemedium and being configured so as when executed on a processor to:control a ratio between an input voltage supplied to the driver circuitand a target output voltage to be output from the driver circuit byvarying a duty cycle of a pulse width modulated signal, and by switchingbetween operating modes of said voltage converter; determine that thedriver circuit is operating in an undesired state caused by acombination of the duty cycle and the operating mode of the converter;and supply the pulse width modulated signal to the resistance adjustmentcircuit to adjust an equivalent output resistance of the driver circuitin response to said determination.